Craig Finseth is author of an excellent textbook on constructing text editors called The Craft of Text Editing: Emacs for the Modern World
(Springer-Verlag & Co., 1991). This book also was translated into Japanese. The title is somewhat of a misnomer. The book is not about how to improve one's skills at "text editing," but rather about programming an editor for input devices, file formats, buffers, text objects, string manipulation, error handling, and design considerations. The author uses Emacs to illustrate some of these principles, which should be borne in mind in future editing projects.
Craig is also author of the well-known List of Emacs Implementations, posted to Usenet every 2 months. The current version of his Emacs distribution list is here: http://www.finseth.com/emacs.html
You can get a copy of his book in HTML, PostScript?, or LaTeX at: